The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with phased array quadrature surface coils for medical diagnostic applications of magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated, that the present invention will also find application with other phased array coil techniques, spectroscopy, imaging for other than medical diagnostic purposes, and the like.
Conventionally, magnetic resonance imaging procedures include the excitation of magnetic resonance of selected dipoles within a subject and receiving the magnetic resonance signals emanating from the dipoles. In many applications, the magnetic resonance signals are received with whole body RF coils, i.e. circularly cylindrical RF coils which surround the patient receiving bore of the magnetic resonance imaging apparatus. In other applications, a surface coil is applied to a surface of the patient adjacent the area of interest while the patient is located within the bore to receive the magnetic resonance signals emanating from the resonating nuclei.
Various surface coil configurations have been utilized. A simple loop, e.g. a simple square or circle, coils have been utilized to receive the magnetic resonance signals. Loop coils are sensitive to signal components that are perpendicular to the loop and insensitive to components in the plane of the loop. In order to improve the signal-to-noise ratio, quadrature surface coils have been utilized to examine a region of interest in quadrature, i.e. receive signal components that are perpendicular to the coil and components that are parallel to the coil. See, for example, U.S. Pat. No. 4,918,388 which includes a loop coil and a flat Helmholtz coil, both of which receive resonance signals from the same region. The loop and flat Helmholtz coils are sensitive to orthogonal components of the magnetic resonance signal. When the output of one of the loop and flat Helmholtz coils is phase shifted by 90.degree. and the two signals are combined, the signal-to-noise ratio is improved by about the square root of 2.
In order to examine larger regions, surface coils consisting of a plurality of loop coils have been used. See, for example, U.S. Pat. No. 4,825,162 of Roemer and Edelstein. More specifically, a series of loop coils are partially overlapped in order to examine contiguous regions. As explained mathematically by Grover in "Inductance Calculations" (1946) and summarized in the Roemer and Edelstein patent, the mutual inductance between adjacent coils is minimized when the coils are positioned with a slight overlap. Although the use of overlapped loop coils with the induction minimized enabled a larger area to be examined, each coil was linear, i.e. only sensitive to the perpendicular component. No quadrature detection was provided.
Another surface coil technique for examining a larger area includes positioning a series of coils in a partially overlapping relationship. However, instead of selecting an overlap which minimized the mutual inductance between like loop coils, alternate linear coils are 90.degree. out of phase from each other, i.e. orthogonal, to eliminate the mutual inductance. See U.S. Pat. No. 4,721,913 of Hyde, et al. In the Hyde, et al. patent, the series of linear coils were arranged contiguous to each other to examine partially overlapping regions. The polarity of the coils alternates between orthogonal component sensitive coils and parallel component sensitive coils.
The present invention provides a new and improved quadrature multiple coil array which overcomes the above-referenced problems and others.